US warns Israel PM not to 'betray' trust on nuclear deal

US Secretary of State John Kerry Monday warned Israel's prime minister against revealing details of an emerging Iran nuclear deal on the eve of the Israeli leader's speech to the US Congress.

While he did not mention Benjamin Netanyahu by name, Kerry told reporters in Geneva he was "concerned by reports" that "selective details" of the deal aimed at curbing Tehran's nuclear programme would be revealed in the coming days.

His deputy spokeswoman Marie Harf went further, saying discussing the contents of classified briefings by US officials to Israelis would "betray" America's trust.

"We've continuously provided detailed classified briefings to Israeli officials to keep them updated and to provide context for how we are approaching getting to a good deal," she told reporters in Washington.

"Any release of any kind of information like that would, of course, betray that trust."

The comments come after an Israeli official said the Jewish state knew about the emerging agreement and that the prime minister would elaborate in his congressional address.

Kerry launched a series of talks with his Iranian counterpart Mohammad Javad Zarif in the Swiss lakeside town of Montreux late Monday as they seek to pin down a deal by a March 31 deadline.

"The best way to deal with the question surrounding this nuclear programme is to find a comprehensive deal, but not a deal which comes at any cost," he told reporters.

"We have made some progress, but we still have a long way to go, and the clock is ticking."

The P5+1 group of world powers that are negotiating with Iran have only a few more weeks to reach a political framework for a deal, with the final technical details to be arrived at by June 30.

But Israel is worried the deal will ease sanctions on Tehran -- which is what Iran wants -- without applying sufficiently stringent safeguards to stop Iran acquiring enough fissile material to develop an atomic bomb.